Slaugth
Remember when you played 40k and thought it was all 'sunshine' and 'rainbows' yea tyranids are mean and their Genestealer cult babies are works of art that would make H.P. Lovecraft blush. "Ooh but the warp is literal nightmare fuel" you say, yea but the warp is just full of people who were neglected by their parents, addicts after cutting crackers out of his diet, grumpy puppies, naked ladies, and Florida residents. Nothing of real fright...and then these fuckers get written into existence.
Like a fever nightmare that crawled out of the dreams of someone who has Ptsd from when they had jiggers on them (Don't look them up please dear god!). The Slaugth are a literal swarm of maggots that eat dead bodies (Guess how they get the bodies dead in the first place) although their favorite part of the body to chew on are the brains. When the Slaugth do eat brains they actually absorb its information and memories to find new places to inhabit and new victims to devour.
Overview
The Slaugth are aliens that look like swarms of maggot wearing black robes and to make them perform a hostile takeover of your dreams tonight, uses masks to hide their 'beautiful faces'. Because of their nature, they like to stay hidden in obscure places like deep within the lowest parts of hive worlds or your mom's basement, they are ancient but it's unknown how old they actual are or where their homeworld is. They also like too give tech to greedy politicians and governors as a way to cause political disarray as a way to soften up the PDF.
Status
Nowadays the Slaugth and their fans are praying to Yog Sothoth that they get their own codex, but the Slaugth, along with other minor races such as the Rak'gol and the Enoulians were the brain-children (pun intended) of Fantasy Flight Games, which puts their canon status into question. Not to mention the whole Worm-That-Walks thing is long established horror trope that has been used in other RPGs before. Chances are that it will only happen when Games workshop truly runs out of ideas money.
In all honesty with all the obscure factions like genestealer cults and skitarii getting their own armies it wouldn't be surprising of the Slaugth get some minor cameo in one of the armies. The Calixis Sector and the Koronus Expanse are being obliquely mentioned in the rulebooks these days. So who knows? More likely, better known and fluff embedded races like the Hrud or Demiurg will get a codex first.